Electromagnetic waves have been used in various applications to apply energy to objects. In the case of radio frequency (RF) for example, electromagnetic energy may be supplied using a magnetron, which is typically tuned to a single frequency for applying electromagnetic energy only in that frequency. One example of a commonly used electromagnetic device is a microwave oven. Typical microwave ovens apply electromagnetic energy at the single frequency of 2.45 GHz. To increase the distribution of electromagnetic waves, the typical microwave oven includes a metallic fan (behind a grill in the oven) to disturb the standing wave pattern and in an attempt to achieve more uniform energy distribution in the oven's cavity.
Due to the nature of the absorptive properties of electromagnetic energy, even if uniform electromagnetic field distribution could be achieved at a particular frequency, energy absorption might not be uniform. This is because differing materials (or materials having varying characteristics) typically have variable absorptive properties. Moreover, absorptive properties are often a function of temperature and/or phase of the materials in the object. Thus, as the temperature and/or phase of an object changes, e.g., due to electromagnetic energy application, the object's absorptive properties may change, and the rate and magnitude of this change may depend on properties of material(s) in the object and the amount of energy required causing those changes. In addition, the shape of an object may contribute to its absorptive properties at a particular frequency. Irregularly shaped objects, for example, may exhibit irregular electromagnetic energy absorption. All these factors can make it difficult to control the absorption of electromagnetic energy in an object.